An additive color mixture method is employed as a general coloring method. According to this method, when light beams of two or more colors impinge on the retina, they are mixed and are perceived as another color. This method serves as the basic principle for current color display devices. R (red), G (green) and B (blue) are employed as three discrete color beams (beams of primary colors that cannot be produced by mixing another two colors). Another method that is used is a subtractive color mixture method for which are employed three complementary color beams, C (cyan), M (magenta) and Y (yellow) that respectively are complements of R (red), G (green) and B (blue). With a color television (CRT: Cathode Ray Tube), which is a typical example device that employs these color mixtures, colors are mixed by the light emitted by a RGB phosphor array. Color liquid crystal display devices are also based on this idea.
FIG. 1 is a specific diagram showing the arrangement of pixels and sub-pixels for RGB colors used to display a color image on a liquid crystal display device (LCD). According to the most popular arrangement, a pixel 10 is divided into three R (red), G (green) and B (blue) segments (an R primary color sub-pixel 11, a G primary color sub-pixel 12 and a B primary color sub-pixel 13), and these segments are driven using signals corresponding to the individual colors to display the color image. In other words, when light beams pass through minute RGB color filters they are mixed. An additive color mixture is obtained by producing a light beam having a single color from a set of minute points of different colors, i.e., by the effect provided by the inability of the human eye to spatially resolve colors. Such a mixture is called “parallel additive color mixture.”
Furthermore, even when the speed in changing colors exceeds the limit of the speed of resolution for the human eye, i.e., when the colors flicker too fast and can not be distinguished, mixed colors can be seen. This is called a “continuous additive color mixture.” This phenomenon occurs as a result of the afterimage characteristic of the eye.
As an additional method, there is a “simultaneous additive color mixture” method that is used for a projection type CRT or LCD that simultaneously projects the three primary colors onto a screen to produce a color mixture. This is an example of the typical additive color mixture phenomenon. According to the principle of simultaneous additive color mixing, when RGB primary color beams are emitted at the same time and to the same space (same location), an additive color mixture is enabled. However, color for a direct-vision CRT or LCD depends on spatiotemporal color mixing.
When an information display device displays motion pictures, in order to replay movements using the afterimage characteristic of the human eye, image data to be displayed must be rewritten (i.e., next data writing performed) using a time unit that is adequately shorter than the afterimage time. When the length of this rewriting time is greater than the response time of the human eye, a person viewing motion pictures experiences a flickering sensation and the movement in the pictures seems clumsy.
To display still pictures, except when pictures are displayed by an information display device having a memory, the display contents must be changed (i.e., next data written) at such a speed that a person experiences no flickering sensation, in accordance with the period display information is held in the information display device.
A display principle that does not require much memory and provides a high response speed tends to be employed for a display device, such as a television or an information processor, that needs to display both motion and still pictures. In this case, refresh frequency and a refresh rate for the display contents tend to be determined while taking into account the sensitivity to flickering of the human eye. Generally, since the refresh frequency is high, a person seldom experiences the flickering sensation and a clear display having a high quality can be provided.
It should be noted that there is a considerable time difference between a “display” period or a “display in progress” period and an image data “writing” period. This difference varies depending on the display principle that is employed. It is natural for the “display” period or the “display in progress” period, which is the result of data writing, be much longer than the “writing” period that is the source of the display. In addition, since the display is provided in response to the data writing, a time lag or delay occurs between the two periods.
As an example, for a liquid crystal display, a capacitor can hold a charge for a period much longer than a signal voltage writing period. In other words, the LCD has a specified memory capacity. That is, when an active device, such as a TFT, is provided for individual sub-pixels, the degree of freedom for the adjustment of a capacitor is larger. For a CRT, it appears that the phosphors emit light longer than the writing period. Therefore, the CRT can display, or can continue to display, data substantially longer than the writing period. It is important here that when the rate of data writing is increased, the same effects can be obtained as those that are obtained when the display period is substantially extended. This is a refresh period, and can be understood instinctively.
For a liquid crystal display, the time response speed for a display varies in accordance with the display mode (principle): from several tens to several hundreds of milliseconds at the least, and as the refresh frequency is increased, the degree of freedom for technical selection is reduced. The speed at which the orientation of the liquid crystal sandwiched between electrodes is changed, in accordance with charging/discharging, corresponds to a so-called “drive speed” or “(time) response speed” of liquid crystal. Since some driving power is consumed each time a display is repeated, power is wasted. Therefore, generally, a low refresh rate is better, as long as there is no deterioration of the display quality.
Therefore, the refresh rate for an information display device is determined while taking into account the price of the device, power consumption, and the sensitivity to flickering of the human eye.
There is a “color field sequential method” that employs a “continuous additive color mixture.” This is a color display method that uses color mixing in a time sharing manner. This color technique originated at the beginning of the TV age, and is still being improved on as part of the development of the liquid crystal technique. The color field sequential method is sometimes called a “time-sharing method,” but this may be misunderstood as time-sharing driving (dynamic driving), and for this specification, the color field sequential method that is popular as the TV method is employed.
In the United State, the color field sequential method was employed for color broadcasting from 1950 to 1953. However, since the color TV standard method was standardized, as an NTSC method that is compatible with the monochrome method, development of the color field sequential method was halted. The color field sequential method, however, has potential merits in that it is superior to other methods in hue replaying and that high resolution is available, due to the fact that a shadow mask is not required for a CRT, and because as only ⅓ of a pixel is required for an LCD, thereby allowing the number of pixels to be increased three times.
In FIG. 2 is shown a drive principle for the color field sequential method. In the prior art, one color image on the left side is divided in a time sharing manner at the same intervals as for RGB colors; image segments for each color are repetitiously displayed from the top (from the top to the bottom in FIG. 2) in the order red (R), green (G) and blue (B); and at least three colors are employed to complete one picture.
Since the display contents must be changed (i.e., next data writing) in the order R, G and B, pictures for G and B must be displayed in a period extending from the time an R picture is displayed until the next R picture is displayed. For an R picture, accordingly, flickering is noticeable compared with R pictures that are constantly displayed. Therefore, for each color a division count for the time unit is considered to be the measurement that indicates the flickering frequency (normally, a time sharing count is employed but another count can be employed), and it is important that eye sensitivity be evaluated in accordance with the division count.
The most common measurement used for indicating a division count for the time unit is a “scanning frequency.” The scanning includes as a broad definition the analyzation or assembly of image data. The scanning is required for repetitiously writing on the display device.
It is well known that the sensitivity of the eye to flickering light is affected by the frequency of the flickering and the brightness of the light. The relationship between white light and the frequency of flicking was also the subject of an experiment into the prior art. In FIG. 3 are shown the results of the experiment conducted for the prior art. In this example, the discrimination threshold was measured for stimulus light where luminance was changed as a sinal wave as time elapsed. The vertical axis represents the modulation amplitude, and shows that the degree of modulation increased from the top to the bottom. The horizontal axis represents a modulation frequency and shows the threshold values obtained when retinal illuminance is changed from 77 to 850 and to 9300. The results shown are for a bandpass type filter, with the frequency at the peak increasing as the retinal illuminance was increased. The shielding high-frequency increased as the modulation degree increased.
When the flickering frequency of stimulus light is low, a so-called flicker occurs and the degree of brightness changes as time elapses. When the flickering frequency is high, so-called fusion occurs and the degree of brightness appears to be uniform. The flickering frequency that corresponds to the critical point is called the “critical flicker frequency (CFF).” According to what is known as the Ferry-Porter rule, within a specific frequency range the CFF (Fc or FC) is proportional to the logarithm of luminance L of stimulus light. This relationship is represented by the following equation.Fc=a×log(L)+b (a and b are positive constants)  (1)
As is apparent from the results shown in FIG. 3, the frequency at the peak increased as the illuminance was increased, thereby proving the Ferry-Porter rule. As is easily understood from the above equation, the CFF is lowered as the luminance L is reduced. However, this rule is merely obtained for so-called white light that is produced as the result of the additive color mixture, and is not obtained by R, G and B decomposition.
The relationship between CFF (Fc or CF) and the frequency has not yet been analyzed for each color R, G and B. The prior art treats the three primary colors equally. Since the CFF is an indicator for a frequency that constitutes a refresh frequency required for a time unit, it is very important that the CFF for each color be precisely ascertained. This is because since the spectral luminous efficacy affected by the brightness differs for the R, G and B colors, the flickering frequency may also differ for the R, G and B colors.
If such a condition exists, it would be not economical for a flickering frequency that is higher than necessary to be provided for a color that a person does not perceive very well. In addition, a person will visually feel uncomfortable if a satisfactory refresh frequency is not provided for a color that the person perceives well. This is important for a color sequential method whereby R, G and B data must be sequentially written.
It is, therefore, one object of the present invention to provide a color display method whereby flickering frequencies for RGB primary colors are not treated equally, while taking into account the fact that the sensitivity of the human eye relative to a flickering frequency differs for each color.
It is another object of the present invention to provide a matrix driving liquid crystal display module that is appropriate for the above color display method, and a PC system that includes such a module.
It is an additional object of the present invention to provide a double-panel projection type display device that is appropriate for the above color display method.